This invention relates to lasers and more particularly to a method for cavity dumping Q-switched radiation from a molecular laser to obtain pulses having submicrosecond duration with a high pulse repetition frequency.
The generation of pulses from a carbon dioxide laser having durations in the range of a few nanoseconds to three hundred fifty nanoseconds suitable for laser radar communication type applications is highly desirable. Pulses having this duration are difficult to achieve in part from the recognized difficulty of modulating carbon dioxide lasers in general and the fact that this range of pulse duration lies between two common techniques for modulating infrared molecular lasers: mode locking and intracavity Q-switching. The pulse duration of interest lies in the intermediate range when neither mode locking nor intracavity Q-switching would generate pulses which are of the desired duration. Mode locking techniques produce pulses of a few nanosecond duration which are too short for present day radar scenarios and have only a fixed pulse repetition frequency which may be unsuitable for many communication applications. Intracavity Q-switching techniques, either by saturable absorbers or by an electrooptical modulator, have difficulty producing pulses of less than three hundred nanoseconds at the full width half maximum point due to the dynamics of the upper energy level in the gain medium. A copending application entitled "Apparatus and Method for Cavity Dumping a Q-Switch Laser" Ser. No. 872,282 filed on even date herewith and held with the present application by a common assignee, discloses a method for the generation of short laser pulses from a laser having a gaseous gain medium operating in a continuous wave mode with the laser pulses having a pulse width capable of being varied from fifty to three hundred fifty nanoseconds at a pulse repetition frequency of up to twenty-five kilohertz utilizing a modulator incorporating a Stark effect gas.
For applications requiring pulses of laser radiation having moderate power and high pulse repetition frequencies, passive Q-switch saturable absorbers have been employed with carbon dioxide lasers. Skolnick et al in U.S. Pat. No. 3,764,937 filed Apr. 26, 1972 and held with the present application by a common assignee discloses a SF.sub.6 saturable absorber to passively Q-switch a carbon dioxide laser having intracavity dispersive elements to prevent laser oscillations at lines for which the SF.sub.6 has a low or zero absorption and to select the operating wavelength of the laser. In addition the cavity length is controlled to stabilize the output to a given pulse repetition frequency. For this configuration the intrapulse period is a function of the recovery time of the inversion of the carbon dioxide gain medium while the pulse tail is a function of the recovery time of the saturable absorber. A long pulse tail is undesirable for pulsed laser radar applications.
The generation of high peak power, temporally short (tens of nanosecond) pulses from a carbon dioxide laser cannot be easily achieved by passive Q-switching. Additionally, with passive Q-switching, the pulse repetition frequency is determined by the active medium and the saturable absorber medium dynamics. Active modulation offers the alternative of obtaining electronically controllable pulse widths and pulse repetition frequencies. Active intracavity modulation and Q-switching of a carbon dioxide laser can be accomplished in very short times at high rates with gaseous and solid crystalline modulators. However, fast switching to remove a high loss from the laser is not by itself sufficient to efficiently generate short high peak power pulses. Day et al in the IEEE Journal of Quantum Electronics, Vol. QE-6, No. 9, September 1970, in an article "Electrooptic Q Switching of the CO.sub.2 Laser" discloses the utilization of an electrooptic modulator to Q-switch a molecular laser. A sufficient voltage is first applied to the modulator to eliminate continuous wave oscillations within the laser. The voltage is then pulsed to zero to Q-switch the laser with the Q-switched pulse passing through an output mirror of the laser cavity. A gas absorption cell was inserted into the cavity to limit the bands of radiation oscillating within the cavity. Q-switched pulses having pulse widths of at least two hundred nanoseconds were obtained.
Bridges et al in Applied Physics Letters, Vol. 14, No. 9, May 1, 1969 in an article entitled "Spontaneous Self-Pulsing and Cavity Dumping in a CO.sub.2 Laser with Electro-Optic Q-Switching" discloses a technique for obtaining spontaneous self-pulsing from a carbon dioxide laser using an intracavity polarization coupler and an electrooptic crystal to Q-switch and cavity dump the intracavity radiation to obtain an output pulse having a pulse width of twenty nanoseconds. A high voltage is first applied to the modulator to obtain a high coupling loss condition. Q-switching to obtain a buildup of the optical flux within the cavity in the form of spontaneous self-pulsing is achieved by pulsing the voltage to zero. A pulse having a pulse width of twenty nanoseconds may then be dumped from the cavity by reflection from the polarization coupler, when the voltage is pulsed back to its original high DC level.
A laser beam transmitter having a moderate to high average power and which is efficient, frequency stable, capable of being pulsed in a variety of formats, as well as capable of achieving high peak powers, short pulse widths, and high pulse repetition rates with high interpulse and intrapulse frequency stability is desirable for radar systems. Implementation of such a pulsed laser transmitter in a coherent or heterodyne infrared radiation radar would provide a system capability for simultaneous high range and range rate measurement accuracy and the ability to engage multiple targets. Present methods of obtaining output pulses are not capable of producing pulses with pulse widths controllably variable from twenty to three hundred nanoseconds nor can they provide pulses at a high pulse repetition frequency.